Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, this system has advantages over silver halide photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
Recently, various printers allowing higher-speed printing have been developed and commercialized increasingly in the field of the dye-diffusion transfer recording systems. The high-speed printing is a performance desirable for shortening the time of the user waiting for printing in photo shop.
For high-speed printing, it is needed to make the dye transferred from heat-sensitive transfer layers of the heat-sensitive transfer sheet in a shorter time, but it also leads to deterioration of storage properties of the heat-sensitive transfer sheet at the same time. Accordingly, it is necessary to raise the transfer density without deterioration of the storage properties of the heat-sensitive transfer sheet. From the viewpoint, known are methods of using multiple dyes and multiple binders in the heat-sensitive transfer layer (e.g., JP-A-4-148990 (“JP-A” means unexamined published Japanese patent application) and JP-A-6-286335).
Further intensive studies with the methods above along with the need for further higher-speed printing revealed that it was indeed possible to raise the maximum density by the methods, but also that the methods raised a new problem that the balance of colors in the image, yellow, magenta and cyan, in a region lower in density was significantly degraded. The color balance in print image, especially in neutral-colored gray image (e.g., cloud in cloudy sky) or in female portrait image, should be kept favorably. It may be effective, to solve the problem, to change the heat quantity of the thermal head and thus change the amount of the transferred dyes to a desirable density by modifying the lookup table for adjustment of the electric signal. However, the method unfavorably demands correction of the lookup tables in all printers the users are already using.